1. Field of the Invention
This invention relates to the field of multilayered structure fabrication. More particularly, this invention relates to a multilayered structure and a method for fabricating a multilayered structure from layers of green-tape, wherein an adhesive is applied to the layers of green-tape to bind the layers together.
2. Description of Related Art
Multilayered structures, such as multilayered ceramic structures, find application as electronic devices, such as ceramic capacitors, multilayered ceramic integrated circuits (MCIC), multichip modules, integrated circuit packaging, high temperature sensors (such as exhaust gas sensors), fuel cells, and fuel cell reformer systems. Multilayered structures also find application as microchannel devices, such as secondary electron multipliers and microfluidic devices.
Such multilayered structures are often made by laminating together layers of green-tape, typically ceramic green-tape, and then firing the laminated layers to form the finished structure. The manufacturing, processing, and applications of ceramic greentapes are described generally in Richard E. Mistler, xe2x80x9cTape Casting: The Basic Process for Meeting the Needs of the Electronics Industry,xe2x80x9d Ceramic Bulletin, vol. 69, no. 6, pp. 1022-26 (1990), and in U.S. Pat. No. 3,991,029, which are incorporated herein by reference.
In electronic applications, one or more of the green-tape layers may include metallized portions to provide conduction pathways for electrical current in the finished multilayered structure. For microchannel or microfluidic devices, the green-tape layers may also have portions punched out to define vias, channels, or cavities, which may define fluid pathways.
Conventional lamination processes generally require lamination at high pressure. Typically, the lamination process occurs in two steps. First, the stacked layers of green-tape are subjected to about 1000 to 1500 psi in a uniaxial press. Next, the layers are subjected to about 3000 to 5000 psi in an isostatic press for about 10 to 15 minutes at an elevated temperature, such as 70xc2x0 C.
This conventional lamination process has a number of limitations. For example, the relatively long period of time required in the isostatic press is undesirable in a large scale manufacturing process.
Additionally, such high pressures tend to distort the dimensions of internal structures present in the layers and can damage certain materials and devices, which may be desired to be included in the finished multilayered structure. Control over the dimensions of internal structures is also undesirably low when such high pressures are used. The problem is particularly acute when structures such as internal or external cavities or channels are desired to be formed in the multilayered structure, as such structures tend to close up when such high pressures are applied. The problem is more acute the larger the cavity or channel is. Accordingly, the formation of all but the very smallest internal cavities and channels, i.e., those with sizes less than about 20 microns, in the multilayered structure is very difficult to do reliably when such high pressures are used for lamination.
In a first principal aspect, the present invention is a method for fabricating a multilayered structure from a plurality of green-tapes, wherein a room-temperature adhesive, i.e., an adhesive that has a glass transition temperature below room temperature, is applied to the green-tapes. The green-tapes are then stacked together to form a multilayered green-tape structure. The room-temperature adhesive binds the green-tapes by penetrating the surfaces of adjacent green-tapes. The multilayered green-tape structure is then fired to achieve a predetermined sintering temperature for a predetermined amount of time to form the multilayered structure.
In a second principal aspect, the present invention is a method for fabricating a multilayered structure from a plurality of green-tapes, wherein an adhesive having a polymer different from the polymer binder of the green-tapes is applied to the green-tapes. The green-tapes are stacked to form a multilayered structure, which is then fired to achieve a predetermined sintering temperature for a predetermined amount of time to form the multilayered structure.
In a third principal aspect, the present invention is the multilayered structure fabricated by these methods.
In another aspect, the present invention provides a method for fabricating a substantially monolithic structure from a plurality of green-tape layers including at least a first green-tape layer and a second green-tape layer. The method includes applying an adhesive to a first surface of said first green-tape layer, wherein said adhesive has a glass transition temperature below room temperature. The method further includes laminating said first or second green-tape layer with at least a third green-tape layer, including applying a pressure exceeding 2500 psi, prior to contacting said second green-tape layer to said first green-tape layer, such that said adhesive interposes said first surface of said first green-tape layer and a second surface of said second green-tape layer, thereby binding said first green-tape layer to said second green-tape layer. The method optionally includes repeating the applying and contacting steps to add additionally green-tape layers to the multilayered green-tape structure. The green-tape structure is laminated at a pressure of less than 2500 psi and fired, thereby forming said substantially monolithic structure.
It is the primary an object of the present invention to provide a method for fabricating a multilayered structure without the application of high pressures.
It is also an object of the present invention to provide a method for fabricating a multilayered structure that is practical to apply in a large scale manufacturing operation.
It is a further object of the present invention to provide a method for fabricating a multilayered structure that minimizes deformation of and damage to internal structures.
Still another object of the present invention is to provide a method for forming cavities and channels in a multilayered structure with good dimensional tolerances.
Yet another object of the present invention is to provide a multilayered structure that includes internal electrical and fluid pathways and a method for fabricating such a structure.